Displacive disorder, antisite disorder and magnetic properties of cubic pyrochlores Bi2MM’O7 with M=Sc, Cr, Mn and Fe and M’= Nb, Ta and Sb

M. C. BLANCO; D. G. FRANCO; Y. JALIT; E. V. PANNUNZIO-MINER; E. V. PANNUNZIO-MINER; J. A. PAESANO; G. NIEVA; R. E. CARBONIO

Buenos Aires. Argentina

Congreso; At the Frontiers of Condensed Matter V; 2010

Pyrochlore oxides are being the subject of renewed interest because of their many interesting applications. Specifically those with formula A2MM´O7 were recently informed to be alternative cathodes for IT-SOFCs [1]. Proton conductivity has also been observed in doped Sm2Sn2O7 [2] and dielectric properties were informed for doped Bi1.5Zn0.92Nb1.5O6.92 [3]. Many other important applications make the synthesis of new pyrochlore oxides and their fundamental studies a very interesting area of research. The typical pyrochlore compound has the general formula A2B2O7, with a cubic crystal structure Fd-3m (SG 227), and it is well described as A2B2O6O’ comprising two interpenetrating sublattices with the formulae A2O’ and BO6. Non spherical ions (like Bi3+) inside the classic cubic structure can be accommodated through displacive disorder. This disorder results in the appearance of hkl reflections where h = 4n, k = 4n, and l = 2n. Its appearance is rationalized by a static displacement of the A ions toward the surrounding ring of O atoms. The synthesis of new Bi2MM´O7 pyrochlores with a combination of both, a magnetic ion and a spectator ion on the M sites (where M = Sc, Cr, Mn and Fe and M’ = Nb, Ta and Sb) have been selected as an interesting family to study their fundamental properties. They were successfully synthesized by solid state method, from stoichiometric quantities of the corresponding binary oxides and appropriated thermal treatments. XRD patterns, combined with Mössbauer spectroscopy for Fe containing compounds, allowed establish the displacive disorder and site disorder presence for all compounds. Their structures were refined by Rietveld analysis of PXRD patterns, and the comparison between the ideal cubic and displaced cubic pyrochlores was carried too. Resulting cell parameters are consistent with the M3+ cation in high spin configuration in octahedral coordination. Magnetic measurements were made in a commercial SQUID in the 10-300 K temperature range, and they allowed observe a wide range of magnetic characteristics, starting by expected diamagnetism for Sc3+ pyrochlores, and indications of magnetic frustration for Fe3+ pyrochlores.   [1] R. Martínez-Coronado, A. Aguadero, C. de la Calle, M. T. Fernández and J. A. Alonso, J. Power Sources (2010) (In press). [2] K. E. J. Eurenius, E. Ahlberg, C. S. Knee, K. E. J. Eurenius, E. Ahlberg and C. S. Knee, Solid State Ionics (2010) (In press). [3] A. Mergen, H. Zorlu, M. Özdemir and M.Yumak, Ceramics International (2010) (In press).